Position-measuring devices for measuring the angular position of a shaft are familiar from a multitude of publications. Such position-measuring devices are referred to as rotary encoders. Moreover, if a position-measuring device is arranged such that, in addition to the angular position of the shaft, it is also possible to measure the number of revolutions covered, it is referred to as a multiturn rotary encoder.
In principle, two approaches are conventional for implementing the multiturn unit, e.g., the unit for determining the number of revolutions covered by the shaft. For example, there are multiturn units based on gears, and there are multiturn units based on counters.
In the case of gear-based multiturn units, the input shaft actuates one or perhaps several gear stages which gear down the input speed. Given a gear stage having a reduction ratio of 16:1, for example, the output shaft of the gear stage rotates one time per 16 revolutions of the input shaft. The output shaft in turn drives a code carrier, whose angular position allows conclusions about the number of revolutions of the input shaft per unit time.
An example for a gear-based multiturn rotary encoder is described in German Patent No. 28 17 172, which relates to a multi-stage incremental shaft encoder having a first angular-increment code disk and a plurality of downstream angular-increment code disks.
European Published Patent Application No. 1 457 762 describes a device for measuring the position, the displacement, or the rotational angle of an object. It includes three measuring standards in the form of three successive code disks, which are coupled via a differential toothed gear. The code disks are scanned by a scanning unit radially covering the code tracks of all code disks.
Counter-based multiturn units ascertain the number of revolutions covered by a shaft by counting the revolutions of a code carrier which is driven directly by the shaft, and therefore covers the same number of revolutions as the shaft to be measured. Located on the code carrier is a code, which is scanned by a scanning unit. Based on the position signals ascertained by the scanning unit, counting signals are generated in counter electronics for a counter which counts, as a function of the direction of rotation, the number of complete revolutions of the code carrier, and therefore of the shaft. In order to store the counter reading of the counter even when the main power supply is switched off, for example, when the machine in which the multiturn rotary encoder is operated is switched off, and moreover, to maintain the counting function, counter-based multiturn units are frequently equipped with a battery which takes over the energy supply for at least the multiturn unit of the rotary encoder in the event of a failure of the main power supply.
For example, European Patent No. 1 462 771 describes a multiturn rotary encoder having a counter-based multiturn unit.
While the functioning of the multiturn unit during normal operation—that is, when in addition to the multiturn unit, a single-turn unit for measuring the angular position within one revolution of the shaft is also active—can be checked by the single-turn unit (the number of revolutions measured by the multiturn unit must of necessity change when the single-turn unit determines one complete revolution), a failure of the multiturn unit remains undetected in the event the main power supply is switched off.